No completions for the `#[diagnostic]` namespace are provided

[weiznich]

It seems like rust-analyzer currently does not provide any completions for the #[diagnostic] attribute namespace. There are currently two attributes in this namespace: #[diagnostic::on_unimplemented] and #[diagnostic::do_not_recommend]. It would be great if rust-analyzer knows about them and would offer completions if you e.g. type #[diagnostic::. For the later I would expect to get suggestions for both attributes.

[Veykril]

Needs a new module here https://github.com/rust-lang/rust-analyzer/blob/e65dddaf594e66e95d513134f89ffe58f121811b/crates/ide-completion/src/completions/attribute.rs#L1-L408